The present invention relates generally to semiconductor processing systems, and more particularly to a pyrometer system for measuring a wafer temperature employed in a rapid thermal processing (RTP) tool.
Thermal processing furnaces have been widely known and used for many years to perform a variety of semiconductor fabrication processes, including annealing, diffusion, oxidation, and chemical vapor deposition. As a result, these processes are well understood, especially with regard to the impact of process variables on the quality and uniformity of resulting products. Thermal processing furnaces typically employ either a horizontal-type furnace or a vertical-type furnace.
Both conventional types of furnaces are designed to heat semiconductor wafers to desired temperatures to promote either diffusion of implanted dopants to a desired depth while maintaining substantially small line widths (e.g., smaller than 1 micron), or to perform other conventional processing techniques, such as the application of an oxide layer to the wafer or deposition of a chemical vapor layer to the wafer. The heating requirements of the wafer during processing are known and well understood, and therefore are closely monitored.
Conventional vertical-type thermal processing furnaces, such as tube furnaces, are designed to support the processing tube within the furnace in the vertical position. The thermal furnace also typically employs a wafer boat assembly that is mounted to appropriate translation mechanisms for moving the wafer boat into and out of the processing tube. A wafer-handling assembly is deployed adjacent and parallel to the wafer-boat assembly to transfer the semiconductor wafers from wafer cassettes to the wafer-boat assembly. The wafers are then raised into a quartz or silicon heating tube. The tube is then slowly raised to the desired temperature and maintained at that temperature for some pre-determined period of time. Afterwards, the tube is then slowly cooled, and the wafers are removed from the tube to complete the processing. A drawback of this processing technique is that it places constraints on the time-at-temperature to which a wafer can be subjected.
As the critical dimensions for silicon integrated circuits are continuously scaled downward into the sub-micron regime, requirements for within wafer temperature uniformity and wafer-to-wafer temperature repeatability become more stringent. For example, in 0.18 micron technology, the required wafer-to-wafer temperature repeatability is in the order of about +/xe2x88x923xc2x0 C.
Pyrometry has been one method of choice for non-contact temperature measurements of a silicon wafer during processing in a thermal processing furnace. Pyrometry is based on the principle that all objects at temperatures above absolute zero emit electromagnetic radiation as a function of temperature in accordance with Planck""s equation. Based upon that relationship, the temperature of an object may be determined from a distance by measuring its emitted radiation. However, the spectral emissivity value of the surface being measured must be known to calculate the actual temperature. Typically, silicon wafers have backside layers that can drastically alter the spectral emissivity of the wafer through interference effects, which can lead to temperature measurement errors during processing. Furthermore, the emissivity of the wafer is also dependent on the backside surface roughness and wafer temperature. All of these drawbacks make the determination or prediction of wafer emissivity a difficult task.
One technique employed to accurately measure the wafer temperature using pyrometry comprises modified single-color pyrometry with wafer emissivity compensation. An exemplary prior art pyrometry system using such compensation is illustrated in prior art FIG. 1, and designated at reference numeral 10. The single-color pyrometer 10 includes an elevator tube 12 (not shown to scale) having a spider collar 14 coupled to a top portion 16 thereof. The spider collar 14 has several legs 18 (e.g., three (3)) and holds a wafer 20 at a predetermined distance from the tube 12. The spider collar 14 may further hold an edge ring (not shown) that may be employed for wafer edge temperature uniformity control.
A pyrometer head 22 is coupled to a bottom portion 24 of the elevator tube 12. The pyrometer head 22 contains an optical system (e.g., a number of lenses and apertures) that facilitates a flash emission to the wafer 20 and receipt of emitted and reflected light along or parallel to an optical axis 26. The head 22 operates in conjunction with a radiometry channel 28 and a reflectometry channel 30 which communicate signals 32 to a processor 34 for determination of the wafer temperature.
In addition, the pyrometry system includes a port 36 coupled to the elevator tube 12 for the introduction of cooling gas 38, such as nitrogen, thereto through, for example, a supply line 40. Because a bell jar (not shown) is typically used to heat the wafer 20, non-uniform heating of the wafer can occur, causing a center portion thereof to become hotter than peripheral areas. The elevator tube 12 delivers the cooling gas 38 to a center portion of the wafer 20 via the port 36 to assist in temperature uniformity thereat.
In operation, the pyrometry system 10 employs the radiometry channel 28 and the reflectometry channel 30 to determine the wafer temperature in the following exemplary manner. The radiometry channel 28 records the intensity of radiation emitted from the wafer 20 as well as radiation originating from stray light from the bell jar (not shown) and reflected from the wafer. The reflectometry channel 30 records the reflection intensity associated with a flash generated by the pyrometer head 22. The channels 28, 30 deliver the emission data and reflectivity data to the processor 34 that subtracts the stray light from the radiometry signal to obtain the black body intensity of the wafer. The processor 34 further calculates the wafer emissivity by assuming the emissivity is the complement of the wafer reflectivity, and thus extracts the wafer emissivity from the reflectometry signal. The wafer temperature is then calculated or otherwise determined by the processor 34 by dividing the black body intensity by the wafer emissivity.
As seen in prior art FIG. 1, the central optical element of the pyrometer system 10 is the pyrometer head 22. Since the head 22 is mounted at the bottom of the elevator tube 12 to thermally isolate the components therein from the work piece that is at an elevated temperature, the optical path between the wafer 20 and the head 22 is relatively long, for example, about 1 meter. The relatively long separation between the wafer 20 and the head 22 requires that the wafer be extremely carefully aligned such that the wafer normal and the head 22 are aligned with the optical axis 26. Even a small angle or offset of the wafer normal with respect to the axis 26 causes a loss in reflected and emitted signal intensity, thereby adversely impacting temperature calculation accuracy.
For example, as illustrated in prior art FIGS. 2A and 2B, if the wafer 20 is not warped or offset (FIG. 2A) the radiation 40 is fully collected at the bottom 24 of the tube 12 by the head 22, while wafer warpage or offset (FIG. 2B) may cause reflected light 41 that results in signal loss. In order to correct the above problem, a lens 42 is inserted at the top portion 16 of the elevator tube 12 and is held in place via, for example, the spider collar 14 (not shown) at distance from the wafer that ideally is the lens focal length (e.g., about 50 mm), as illustrated in prior art FIG. 2C. The lens 42 re-collects 44 any reflected light 41 that would otherwise not return to the pyrometer head 22. The lens 42 thus allows or corrects for reflections due to warpage or offset, thus facilitating use of the pyrometer with both polished or roughed backside wafers.
Although the pyrometer system 10 having the lens 42 provides relatively good results, there is a continuing need in the art for further refinements and improvements in pyrometry in order to provide high temperature determination accuracy for rapid thermal process control.
The following presents a simplified summary in order to provide a basic understanding of one or more aspects of the invention. This summary is not an extensive overview of the invention, and is neither intended to identify key or critical elements of the invention, nor to delineate the scope thereof. Rather, the primary purpose of the summary is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
In accordance with one aspect of the present invention a pyrometer system and temperature sensing method is disclosed that overcomes several disadvantages associated with the prior art. In particular, a pyrometer having an elevator tube comprising two tubes in a generally telescopic arrangement is disclosed. A cooling gas is introduced in a fluid passageway between the tubes to provide directed cooling to a portion of the wafer or work piece being sensed. The radiation to and from the wafer during analysis occurs within the inner tube and is substantially isolated from the cooling gas, and thus any stratification of gas within the inner tube is independent of changes in the cooling gas flow rate. Further, the inner tube may be evacuated to substantially prevent stratification therein and thereby further improve the optical path of the pyrometer.
In accordance with another aspect of the present invention, a pyrometer system is disclosed comprising an elevator tube having a pyrometer head associated with a bottom portion and a sleeve associated with a top portion thereof. A lens is coupled to the sleeve and is operable to focus radiation/light from a work piece to the pyrometer head. The lens has a focal length that is related to the distance of the work piece therefrom, and the focal length is a function of temperature. The sleeve that is coupled to the lens has a coefficient of thermal expansion that varies the distance between the lens and the work piece as temperature varies in order to compensate for changes in the focal length due to temperature variations. Accordingly, the lens in conjunction with the sleeve is operable to transmit light from the work piece to the pyrometer head without substantial signal loss, thereby improving pyrometer system accuracy.